Differences between 17" and 18" rims

I currently have 17" rims on 215/40ZR/17 tires, but im thinking of upgrading to 18" on 215/35ZR/18 tires. I heard there is a difference in acceleration and top end speed depending on your tire size. Also, isnt there a rule of thumb that says per every 5ZR rating you go down, its an inch on the rim you go up. This way you keep your same speedometer readings and keep the size the same as stock. For example, if you had 16" rims on 205/45ZR/16 tires. And if you upgraded to 215/40ZR/17 tires and rims. The diameter of the tires wont change.

Someone tell me if im right before i make an ass of my self...........:( :confused: :huh: :biggrin2:

There isnt that much of a hieght difference between those 2 sets of tires they are pretty close.

http://www.miata.net/garage/tirecalc.html